The present invention particularly relates to an acceleration sensor and a pressure sensor in a semiconductor device for converting a displacement into electric signals by utilizing piezoresistance effect inherent to a semiconductor crystal such as silicon.
With the recent development of micro machining technology, a semiconductor acceleration sensor has been manufactured by means of film formation or etching on a semiconductor wafer. (See IEEE Transactions on Electron Devices, Vol. ED)-26, No. 12, December 1979, for example.) FIGS. 3A and 3B are views showing a prior art semiconductor acceleration sensor manufactured according to micro machining technology, wherein FIG. 3A is a top plan view and FIG. 3B is a cross-sectional view. A silicon substrate I is etched to form a cantilever 2 and a dead-weight 3. Here, the cantilever 2 is made thinner than any other portion by etching and deformed with acceleration in a direction indicated by an arrow in FIG. 3B. A quantity of deformation of the cantilever 2 is detected by the piezoresistance effect of a diffused resistor 4a formed on the upper surface of the cantilever 2, and the acceleration is obtained by comparing the obtained quantity with that from a diffused resistor 4b. Here, the diffused resistors 4a and 4b are connected with highly diffused areas 5 and output terminals 8. Further, an upper stopper 6 and a lower stopper 7 are disposed to prevent damage of the cantilever 2, and the overall system is provided on a ceramic board 10.
FIG. 4A is a view showing a semiconductor acceleration sensor according to micro machining technology disclosed in Japanese patent laid-open publication No. 1-302167, wherein a groove portion 35 is formed in the vicinity of a supporting body 9 of the cantilever 2 by etching to provide a thin portion 36. Diff-used resistors 4c, 4d, 4e and 4f are provided on the top face of the sensor to constitute a bridge circuit 50. The diffused resistors 4c and 4f function as reference resistors and are provided on the top face of the supporting body 9. The diffused resistors 4d and 4e function as variable resistors for detecting a quantity of deformation of the thin portion 36 and are provided at positions orthogonal to the reference resistors. FIG. 4B shows a detection circuit of the device shown in FIG. 4A.
In the prior art semiconductor acceleration sensor, the thin portion 36 shown in FIG. 4A must be formed in order to improve detection sensitivity, thereby deteriorating the overall mechanical strength.
Here, the thickness of the thin portion 36 relates to detection sensitivity, and conditions for composition, temperature and stirring of etchant must be strictly managed in order to obtain an uniform thickness when forming the thin portion 36 by etching, increasing manufacturing steps such as masking pattern formation.
Furthermore, an area of the top surface of the acceleration sensor becomes large, a number of sensors taken out from, e.g., one silicon substrate is limited and the manufacturing cost is hard to be reduced because the diffused resistors 4a and 4b are provided on the top surface of the acceleration sensor and the dead-weight 3 is formed as shown in FIG. 3A. A predetermined value is required for a width of the cantilever 2, i.e., the width from the front side to the other side in the drawing in order to maintain the strength of the thin portion 36 in the acceleration sensor in FIG. 4A. Therefore, an area of the top face of the sensor can not be reduced as in the acceleration sensor shown in FIG. 3A to limit a number of sensors taken out from the semiconductor wafer, and the cost down is difficult.
In addition, since the diffused resistors 4 for detecting the acceleration are provided on a surface to which acceleration of the acceleration sensor is applied, the diffused resistors 4 must be so disposed as to enlarge a difference in resistance value between the reference resistors and the variable resistors provided on the top face of the supporting body 9.
In order to eliminates these problems, it is therefore an object of the present invention is to obtain inexpensive semiconductor acceleration sensor and pressure sensor with which manufacturing is facilitated and a large number of sensors can be produced from one semiconductor wafer.